How does += (plus equal) work?

I'm a bit confused with the += sign. How does it work?

1. 1 += 2 // equals ?

2. and this

   var data = [1,2,3,4,5];
   var sum = 0;
   data.forEach(function(value) {
       sum += value; 
   });
   sum = ?
   

Solution 1:

1 += 2 is a syntax error (left-side must be a variable).

x += y is shorthand for x = x + y.

Solution 2:

1) 1 += 2 // equals ?

That is syntactically invalid. The left side must be a variable. For example.

var mynum = 1;
mynum += 2;
// now mynum is 3.

mynum += 2; is just a short form for mynum = mynum + 2;

2)

var data = [1,2,3,4,5];
var sum = 0;
data.forEach(function(value) {
    sum += value; 
});

Sum is now 15. Unrolling the forEach we have:

var sum = 0;
sum += 1; // sum is 1
sum += 2; // sum is 3
sum += 3; // sum is 6
sum += 4; // sum is 10
sum += 5; // sum is 15

Solution 3:

That is just a short form for:

sum = sum + value;

Solution 4:

+= in JavaScript (as well as in many other languages) adds the right hand side to the variable on the left hand side, storing the result in that variable. Your example of 1 +=2 therefore does not make sense. Here is an example:

var x = 5;
x += 4; // x now equals 9, same as writing x = x + 4;
x -= 3; // x now equals 6, same as writing x = x - 3;
x *= 2; // x now equals 12, same as writing x = x * 2;
x /= 3; // x now equals 4, same as writing x = x / 3;

In your specific example the loop is summing the numbers in the array data.